The invention concerns a method for the continuous casting of thin metal strip in a continuous casting installation, in which metal is discharged vertically downward from a mold, the metal strip is deflected from the vertical direction to the horizontal direction, and the metal strip is supported and/or conveyed and/or plastically deformed by means of a number of pairs of drive rolls. The invention also concerns a continuous casting installation, especially for carrying out the method of the invention.
A method of this general type is known from EP 1 071 529 B1 and WO 2004/065030 A1. In the continuous casting of thin metal strip, liquid metal is fed from above to a mold, from which the preformed metal strip with a still liquid core emerges vertically downward. The strip cools off and solidifies in the direction of conveyance, and as it moves, it is gradually deflected from the vertical direction to the horizontal direction. Several pairs of drive rolls, which support and convey the strip, are provided for this purpose. Provision can also be made for the pairs of drive rolls to carry out a preliminary deformation of the metal strip, i.e., the metal strip is reduced in thickness. After passing through the pairs of drive rolls, the strip then enters a downstream rolling mill, in which the strip is rolled out further.
CSP refers to a combined casting and rolling process for thin slabs with thicknesses that are usually 45-70 mm but occasionally up to 90 mm. The requirements that are being placed on the dimensional stability of the geometry and the mechanical properties of the finished hot-rolled strip are steadily increasing. At the same time, market demand for hot-rolled strip with the least possible final thickness is also rising. The thinner the hot-rolled strip is to be rolled out in the finishing train, the more difficult it is to control the rolling process. The requirements on the control and adjustment systems in the finishing train increase considerably at final thicknesses below 1.5 mm.
The geometry of the slab that is entering the finishing train also has a significant influence on the stability of the rolling process, especially with respect to the profile and thickness taper of the thin slab over the width of the metal strip and its uniformity over the length of the slab. Abrupt changes in the profile or the thickness taper over the length lead to abrupt changes in the state of flatness within the finishing train and thus to instabilities during rolling, which in unfavorable cases can result in strip folding with loss of production (discontinuation of casting). The slab geometry is a direct quality-determining result of the casting process. In accordance with the prior art, there is only the possibility of realizing a certain amount of thickness reduction in the area of the pairs of drive rolls by the rolling process between the drive rolls.
In the prior art, CSP casting machines are furnished with liquid core reduction (LCR) and offer the possibility of altering the thickness taper of the metal strip or the thin slab by means of position-controlled hydraulic cylinders. The profile of the thin slab depends on the rigidity of the segments and the position of the tip of the liquid crater. The deeper the tip of the liquid crater is located in the casting machine, the greater is the ferrostatic pressure and thus, at a presumed constant segment rigidity, the greater is the deflection of the segments and the thin slab profile that develops. In practice, this means that a change in the casting speed changes the position of the tip of the liquid crater, and consequently an altered slab profile is obtained. In addition, the slab profile can be negatively affected by the wear profile of the segment rollers. This effect or this change can lead to considerable difficulties in the subsequent rolling process.
In any case, previously used CSP casting machines generally did not have liquid core reduction. This means that neither the profile nor the thickness taper of the thin slab could be influenced. In this case, the slab geometry depends on the orientation of the segments relative to one another, on the rigidity of the segments, and, finally, on the position of the tip of the liquid crater. Therefore, in casting machines without liquid core reduction, the problems to be expected in the rolling mill are correspondingly greater.
Therefore, so far there has been no possible means in the CSP process by which the geometry of the thin slab can be improved and held constant for the purpose of creating reproducible conditions for the rolling of the metal strip in the rolling mill.